Add tox job to build docs

This commit adds a tox job to build the docs. It calls the setup.py
build_sphinx command.

Change-Id: I84dbd1aaea142bbe45c73df04856e128621143db
diff --git a/tox.ini b/tox.ini
index c1acde9..dd36e60 100644
--- a/tox.ini
+++ b/tox.ini
@@ -84,6 +84,11 @@
 deps = -r{toxinidir}/requirements.txt
        -r{toxinidir}/test-requirements.txt
 
+[testenv:docs]
+commands = python setup.py build_sphinx {posargs}
+deps = -r{toxinidir}/requirements.txt
+       -r{toxinidir}/test-requirements.txt
+
 [testenv:pep8]
 setenv = MODULEPATH=tempest.common.generate_sample_tempest
 commands =